This invention relates to hydrogenation of residual unsaturation in conjugated diene homopolymers and copolymers. The hydrogenation process is most useful for removing residual unsaturation from low molecular weight polymers.
Anionic polymerization of conjugated dienes with lithium initiators, such as sec-butyllithium, and hydrogenation of residual unsaturation has been described in many references including U.S. Pat. No. Re. 27,145. Reduction of residual olefinic unsaturation by at least 95% is readily accomplished for high molecular weight polymers with nickel catalysts while reduction beyond 99% is difficult.
Recent experiments with low molecular weight polymers containing conjugated diene units have shown that the effectiveness of the nickel catalysts for hydrogenating the polymers decreases as molecular weight of the polymer decreases. Furthermore, it is known from U.S. Pat. Nos. 3,994,868 and 4,866,120 that significant hydrogenation of low molecular weight hydrocarbon polymers having two or more terminal hydroxyl groups per molecule could not be achieved using nickel catalysts. Both patents assert that complete hydrogenation was achieved with palladium catalysts, and the '868 patent further asserts superior results using ruthenium catalysts.
Low molecular weight polymers containing saturated dienes are useful a tackifying resins in making adhesives. Such polymers having two or more terminal hydroxyl groups are particularly useful in coating applications such as described in U.S. Pat. Nos. 4,039,593 and 4,507,430 which utilities are incorporated by reference herein.
Incomplete hydrogenation of the low molecular weight polymers containing conjugated diene units makes the polymers prone to degradation by oxidative and crosslinking mechanisms that are often induced by heat or ultraviolet light.
It is an object of the present invention to provide complete hydrogenation of residual olefinic unsaturation in diene polymers using nickel catalysts, particularly diene polymers having terminal hydroxyl groups.